1. Field of the Invention
The present invention relates to an optical recording medium having a plurality of recording layers from which information is read by the irradiation of laser light and a recording and reading method of the optical recording medium.
2. Description of the Related Art
Blu-ray Discs have now become commercially available as high storage capacity optical recording medium. There are two types of Blu-ray Discs, namely, one that has a single recording layer and one that has two recording layers. In the Blu-ray Disc with the single layer, the recording layer is arranged in a position that is 100 μm away from the light incident surface. In the Blu-ray Disc with two layers, a basic recording layer is arranged in a position that is 100 μm away from the light incident surface, as is the case for the single layer type, and the other recording layer is arranged in a position that is approximately 25 μm away from the basic recording layer on the light incident surface side of the disk.
In the Blu-ray Disc with two recording layers, the arrangement of the basic recording layer is the same as that of the single layer described above in order to adhere to Blu-ray Disc standards. Conversely, the interlayer distances between the basic recording layer and the other recording layer is arbitrarily determined in consideration of crosstalk between the layers, a degree of allowance for scratches, dust, or fingerprints on the surface, a corrective range for spherical aberrations of an optical pickup, a tilt margin, and any errors in manufacturing. However, an interlayer distance of 25 μm between the two layers has already become the norm in effect in Blu-ray Disc standards, and it can be expected that various manufacturers will supply Blu-ray Discs with two recording layers having such a distance therebetween in the future (see, for example, Japanese Patent Application Laid-Open No. 2006-313628).
Furthermore, when there is a plurality of recording layers, in other words, when there are three or more recording layers in total including the basic recording layer, the effect of crosstalk, surface fingerprint, and the like is increased so that design of the disc becomes more complicated. It is necessary to increase light reflectivity in an L0 recording layer, which is the layer farthest away from the light incident surface, however, it is also necessary to increase light transmittance in the other recording layers (except for the L0 recording layer). In order to obtain favorable signal characteristics, the L0 recording layer and an L1 recording layer adjoining this L0 recording layer are preferably spaced as far away from each other as possible. If the interlayer distance is too large, the recording layer lying closest to the light incident surfaces comes so close to the light incident surface that it becomes susceptible to fingerprints and the like. Accordingly, in the case of the optical recording medium having three or more recording layers, factors affecting signal quality are complicatedly intertwined with one another, so that it is necessary for each manufacturer to individually design an optical recording medium while making use of proprietary technology and know-how in order to increase the signal quality in recording and reading. Thus, there is a problem in that, at present, it is difficult to share the standard position of each recording layer among a plurality of manufacturers.
However, if each manufacturer arbitrarily determines the position of each recording layer, an optical recording and reading apparatus will not know the position of each recording layer located on the Blu-ray Disc inserted into such. Accordingly, there may be a problem in that it takes time for the apparatus to achieve focus.